Water doe not change the chemistry of fertilizer.
Miracle Grow is like salt, its molecules can absorb water and can be disolved in water.

If its hardened just bust it up with a big hammer and it will be fine.

You can either dry it out and break it up again, or add enough water to dissolve it all and store it as a liquid fertilizer. If you do that keep track of how much water you add in proportion to the fertilizer so that you know how much to use/dilute when you get around to using it. after a while you will start to get a precipitate in the liquid, which isn't a big problem, but you would want to use it within a year or two to keep it from happening.